1. Field of the Invention:
The present invention relates to a marine contra-rotating propeller drive system, and more particularly to a marine contra-rotating propeller drive system which reduces a speed of rotation of an engine and transmits it to two propellers disposed on a same axis.
2. Description of the Prior Art:
In a propelling system of a ship, in the case where an engine serving as a main engine and two propellers are disposed on a same axis and inner and outer double shafts for driving these propellers are both reduced in speed, normally an epicyclic gear is employed.
As shown in a schematic longitudinal cross-section view of FIG. 4 and in a schematic view taken along line V--V in FIG. 4 as viewed in the direction of arrows of FIG. 5, heretofore, such an epicyclic gear is constructed as a reversing mechanism 2' for reversing rotation of a main engine 1' and transmitting it to the respective propellers.
The reversing mechanism 2' is composed of a sun gear 2B' connected to a main output shaft 7' of the main engine 1', planet gears 2C' meshed with the outer circumference of the sun gear 2B' and rotatably supported via bearings 2D' from a carrier that is integrally connected to an inner shaft 3C' of a rear propeller 6C', and an inner-toothed gear 2A' having its inner circumference meshed with the outer circumferences of the planet gears 2C' and mounted to a front propeller 6A'. In FIG. 4, reference numeral 5' designates bearings for the inner shaft 3C' and an outer shaft 3A'.
In the reversing mechanism 2' constructed in the above-described manner, a torque generated by the main engine 1' is transmitted to the sun gear 2B' connected to the output shaft 7'. Accompanying the rotation of the sun gear 2B', the planet gear 2C' revolves around the sun gear 2B' while rotating on its own axis. Due to these rotation and revolution of the planet gears 2C', the outer shaft 3A' is rotated via the inner-toothed gear 2A' in the opposite direction to the input shaft 7' of the reversing mechanism 2', that is, the output shaft 7' of the main engine 1', and thereby it drives the front propeller 6A'.
On the other hand, the inner shaft 3C' rotatably supporting the planet gears 2C' rotates in the same direction as the input shaft 7' and drives the rear propeller 6C'.
Through the aforementioned operations, the two propellers are driven in the opposite directions to each other, and moreover, at a lower rotational speed than that of the main engine 1'.
However, the marine contra-rotating propeller drive system in the prior art as described above, involves the following technical problems to be resolved:
(1) The planet gears 2C' connected to the rear propeller 6C' and the inner-toothed gear 2A' connected to the front propeller 6A' form a differential gear mechanism because they both rotate about a common axis, hence if a load upon one of the propellers is changed due to an external force, then the rotational speed of the other propeller also varies, thus the rotational speeds of the two propellers would be determined by the both propeller loads, and so, it would become impossible to operate the propellers at optimum speeds. PA0 (2) In the case where the differential planetary rotation is controlled so as to rotate at any arbitrary rotational speeds, contrivances such as providing a brake device at the outer shaft 3A', regulating the revolving speed of the planet gears by means of a hydraulic motor (See Laid-Open Japanese Patent Specification No. 59-96092) or the like, would become necessary, hence the structure would be complicated, and maneuverability of the system would be also deteriorated. PA0 (3) Since the planet gear 2C' revolves, it is extremely difficult to provide a thrust bearing for supporting a thrust acting upon the inner shaft 3C'.
In order to drive at reduced speeds the inner and outer shafts on the same axis while avoiding the aforementioned problems, if parallel-shaft gears are used, then large gears aligned in three or more rows would be necessitated, hence the reversing mechanism would become large-sized, and so, the system would become disadvantageous in view of arrangement and cost.
As another method, it is also possible to make the main engine and the propellers have different axes and to reduce the speed by means of parallel-shaft gears, but in this case also, the system is not favorable in view of arrangement and cost.